In recent years, solid-state imaging devices have been progressing toward a great number of pixels and a narrow pixel pitch, and color mixture or pixel defect that may accordingly occur has a big effect on an image quality.
Defective pixel corrections are generally performed by replacing a signal obtained from a defective pixel with a mean value of signals obtained from the pixels which have the same color as the defective pixel, located around the defective pixel. However, according to the progress in realizing the number of pixels and the narrow pixel pitch, such a general correction method may cause a scar of the correction to be conspicuous. This reason will be described in the following.
FIG. 13 is a diagram showing a pixel arrangement of a solid-state imaging element equipped with a color filter with Bayer pattern. In FIG. 13, the blocks marked with “R” denote R pixels for detecting red light, the blocks marked with “B” denote B pixels for detecting blue light, the blocks marked with “Gr” denote that there are R pixels at left and right sides thereof among G pixels detecting green light, and the blocks marked with “GB” denote that there are B pixels at left and right sides thereof among G pixels detecting green light.
As shown in FIG. 13, in the solid-state imaging element equipped with the color filter of Bayer pattern, the solid-state imaging element includes three types of R pixel, G pixel, and B pixel. The G pixel among the three types of pixels may be divided into two kinds of attributes (Gr pixel, Gb pixel) around which R pixel and B pixel are differently arranged.
Since the Gb pixel and Gr pixel each have a different arrangement of R pixel and B pixel at its adjacent periphery, a difference between a signal level of the Gr pixel and a signal level of the Gb pixel is generated by color mixture depending on the incidence direction of incident light. The larger the incidence angle of light to the solid-state imaging element is like light causing a ghost signal level and the closer to monochromatic light the incident light becomes like red light, the more prominent the difference in the signal level becomes.
Thus, for example, in the case where there is a defective pixel at a position of a Gr pixel (or Gb pixel), if the defective pixel is corrected using four Gb pixels (Gr pixel) which are adjacent to the defective pixel in a X shaped direction (located at an obliquely upper right side, obliquely lower right side, obliquely upper left side, and obliquely lower left side of the defective pixel), since corrections are performed using an output signal of the Gb pixel (Gr pixel) having a different level, it is difficult to have an output level of the defective pixel closer to an original level.
Therefore, it may consider that the defective pixel located at a position of Gb pixel (or Bb pixel) is corrected by using the four Gr pixels (Gb pixels) which are adjacent to the defective pixel in the cross direction thereof (adjacent at the upper and lower, and right and left sides of the defective pixel). However, in this correction method, since the corrections are performed using a signal of a pixel located far from the defective pixel as compared to the method of performing the correction using the four pixels adjacent to the defective pixel in the X-shaped direction, in a high-frequency image, it is difficult to have the output level of the defective pixel close to the original level thereof.
A method of correcting a signal of defective pixel by using signals of pixels surrounding the pixel is disclosed, for example, in the following Patent Documents 1 to 3.
Patent Document 1 discloses a correction method of a defective pixel in a solid-state imaging device equipped with a color filter having Bayer pattern, and the correction method is performed by selecting any one of a first correction processing for performing corrections using output signals of pixels having the same color as the corresponding defective pixel which are adjacent to the defective pixel in a cross direction thereof and a second correction processing for performing corrections using output signals pixels having the same color as the corresponding defective pixel that are adjacent to the defective pixel in a X-shaped direction thereof, based on the correlation between the output signal of the corresponding defective pixel and the output signals of the same colored pixels that are adjacent to the corresponding defective pixel.
Patent Document 2 discloses a method of correcting an output signal of a corresponding target pixel using output signals of the same color pixels that are adjacent to the correction target pixel in the cross direction of the correction target pixel.
Patent Document 3 discloses a method of correcting an output signal of a correction target pixel by using output signals of eight pixels adjacent to the correction target pixel    Patent Document 1: Japanese Patent Application Publication No. 2010-068329A    Patent Document 2: Japanese Patent Application Publication No. 2011-015157A    Patent Document 3: Japanese Patent Application Publication No. H07-336605A
Since the device described in Patent Document 1 is intended to select a correction method based on the correlation between the output signal of the defective pixel and the output signals of the pixels surrounding the defective pixel, there may be a case where the second correction processing is performed even for the high-frequency image, in this case, a scar of the correction to the high-frequency image is conspicuous.
The device described in Patent Document 2 is intended to perform only the processing for correcting the output signal of the corresponding correction target pixel using the output signals of the same color pixels that are adjacent to the correction target pixel in the cross direction of the correction target pixel, it is impossible to solve the problem that a scar of the correction to the high-frequency image is conspicuous.
Like the solid-state imaging device shown in FIG. 13, in the case where the correction method described in Patent Document 3 is applied to the solid-state imaging element having a plurality of types of same-color pixels (Gr pixel, Gb pixel) around which pixels are arrayed differently, correction data are generated using the signals of different colors and a scar of the correction is thereby conspicuous after the correction of the defective pixel.
The present invention has been made in an effort to solve the problems, and an object of the present invention is to provide an imaging device and a defective pixel correction method capable of improving the accuracy of a defective pixel correction.